A person who uses eBay in unfair ways for the sake of his/her own ego.

As a basic way to prevent any business with them, you can spot this kind of people via their feedback page. Usually, these assholes have feedback rating below 99% or a large number of negative feedbacks.
Keep an eye for the feedback comments too.

Another method : copy a desired username and paste it to Google, see the results.

This definition applies to both seller and buyer sides.

Below are the few examples of these eBay Assholes, words in parentheses indicate in which side the asshole on :

1. The ones who refuse to pay after winning a bid or after clicking the "Buy It Now" button. (buyer)

2. The ones who inform that they are currently out of stock to a bid winner so they have to cancel the transaction. (seller)

3. The ones who sell counterfeited goods, imitations, or not-as-decribed goods. The liars are also count as these kind of eBay Assholes. (seller)

4. The ones who received their stuffs in GOOD and AS DESCRIBED conditions, but still drag the seller into disputes, via Paypal or eBay for example, in order to receive their money back. The goods have to be returned though, but fruitless hard efforts are painful, no ? (buyer)
DIGITAL SELLERS, BEWARE ! "Copy-paste" is the reason for this . . .

5. The ones who are hard to communicate with, after dealing a transaction. (seller and buyer)